1. Field of the Invention
The present invention relates to a semiconductor device including a p-type semiconductor layer on an InP substrate.
2. Description of the Related Art
Applications of laser diodes (LDs) include not only light sources for optical disk devices such as CD (Compact Disk) systems, DVD (Digital Versatile Disk) systems and blue-ray disc (BD) systems but also various fields such as optical communications, solid-state laser excitation, material processing, sensors, measuring instruments, medical use, printers and displays. Moreover, light emitting diodes (LEDs) are applied to fields such as indicator lamps for electrical appliances, infrared communication, printers, displays and lighting fixtures.
However, in the LEDs, the efficiency when emitting green light having the highest human visibility is not so high, compared to when emitting light of other colors, and LDs have not yet obtained a practicable characteristic in a visible light range from pure blue (slightly higher than 380 nm) to orange (slightly higher than 600 nm). For example, E. Kato et al. have reported that in a blue-green LD with a wavelength of around 500 nm which is formed by laminating a Group II-VI compound semiconductor on a GaAs substrate, the room-temperature continuous-wave operation for approximately 400 hours with 1 mW has been achieved (refer to E. Kato et al. “Significant progress in II-VI blue-green laser diode lifetime” Electronics Letters 5 Feb. 1998 Vol. 34 No. 3 p. 282-284); however, the material system does not obtain a characteristic exceeding this. It is considered that the reason is attributed to physical properties of the material such as easy formation and movement of crystal defects.
In Group II-VI compound semiconductors, typically p-type conductivity control is not easy, and in particular, there is a tendency that the larger an energy gap is, the smaller the p-type carrier concentration becomes. For example, in ZnMgSSe used as a p-type cladding layer in E. Kato et al. “Significant progress in II-VI blue-green laser diode lifetime” Electronics Letters 5 Feb. 1998 Vol. 34 No. 3 p. 282-284, the larger the Mg composition ratio is, the larger the energy gap becomes. However, when the energy gap is approximately 3 eV or over, the p-type carrier concentration becomes smaller than 1×1017 cm−3, thereby it is not easy to use ZnMgSSe as the p-type cladding layer. It is considered that it is because nitrogen (N) as a p-type dopant in the form of atom is included in ZnMgSSe, but most of nitrogen atoms exist in an interstitial site except for a Group VI site, so the nitrogen atoms are not carriers, that is, it is because the activation ratio of the p-type dopant is low (much lower than 1%). Further, it is considered that the existence of a large number of atoms in an interstitial site in such a manner is a major reason for the formation of crystal defects.
Moreover, in E. Kato et al. “Significant progress in II-VI blue-green laser diode lifetime” Electronics Letters 5 Feb. 1998 Vol. 34 No. 3 p. 282-284, ZnCdSe used as an active layer is not fully lattice-matched to the GaAs substrate, so ZnCdSe has distortion. Typically, in a light-emitting device or a light-receiving device, crystal defects are propagated and diffused from a region having the largest number of crystal defects by influences such as heat, electrical conduction and distortion to reach the active layer, thereby the crystal defects cause deterioration of the device or reduction in lifetime. Therefore, in the case where the active layer has distortion as in the case of E. Kato et al. “Significant progress in II-VI blue-green laser diode lifetime” Electronics Letters 5 Feb. 1998 Vol. 34 No. 3 p. 282-284, when crystal defects exist in the p-type cladding layer or the like, there is a high possibility that the crystal defects cause deterioration of the device. Therefore, even in the case where the energy gap of the p-type cladding layer is increased, it is necessary to be capable of keeping the activation ratio of the p-type dopant at 1% or over, and to form a light-emitting device in which the active layer does not have distortion.
Therefore, the inventers of the present invention and several research groups inside and outside Japan have focused attention on a MgxZnyCd1−x−ySe Group II-VI compound semiconductor (0≦x≦1, 0≦y≦1, 0≦1x−y≦1) as a candidate of a material for forming an optical device emitting light with a wavelength ranging from yellow to green, and have researched and developed the semiconductor (refer to N. Dai et al. “Molecular beam epitaxial growth of high quality Zn1-xCdxSe on InP substrates” Appl. Phys. Lett. 66, 2742 (1995) and T. Morita et al. “Molecular Beam Epitaxial Growth of MgZnCdSe on (100) InP Substrates” J. Electron. Mater. 25, 425 (1996)). MgxZnyCd1−x−ySe (hereinafter simply referred to as “MgZnCdSe”) has such a characteristic that in the case where the compositions of x and y satisfy the following relational expressions, MgZnCdSe is lattice-matched to InP, and when the compositions of x and y are changed from (x=0, y=0.47) to (x=0.8, y=0.17), the energy gap is able to be controlled from 2.1 eV to 3.6 eV.y=0.47−0.37x The composition x ranges from 0 to 0.8 both inclusive.The composition y ranges from 0.17 to 0.47 both inclusive.
Moreover, throughout the above-described composition ranges, the band gap is of a direct transition type, and the energy gap corresponds to a wavelength of 590 nm (orange) to 344 nm (ultraviolet). This suggests that an active layer and a cladding layer constituting a light-emitting device emitting light with a wavelength from yellow to green may be achieved only by changing the compositions x and y in MgZnCdSe.
Actually, it is reported in T. Morita et al. that in the photoluminescence measurement of MgZnCdSe grown on an InP substrate performed by a molecular beam epitaxy (MBE) method, MgZnCdSe Group II-VI compound semiconductors having different composition ratios x and y obtain such a good light emitting characteristic that the peak wavelength ranges from 571 nm to 397 nm (refer to T. Morita et al. “Molecular Beam Epitaxial Growth of MgZnCdSe on (100) InP Substrates” J. Electron. Mater. 25, 425 (1996)).
Moreover, it is reported by L. Zeng et al. that an LD formed of MgZnCdSe achieves laser oscillation by light excitation in each of wavelength bands of red, green and blue (refer to L. Zeng et al. “Red-green-blue photopumped lasing from ZnCdMgSe/ZnCdSe quantum well laser structure grown on InP” Appl. Phys. Lett. 72, 3136 (1998)).
On the other hand, laser oscillation by the current drive of an LD made of only MgZnCdSe has not been reported yet. It is considered that a major reason is that it is difficult to control p-type conductivity by impurity doping into MgZnCdSe.
Therefore, the inventors of the present invention have been conducted research for finding a suitable material for an active layer, a p-type cladding layer or the like in the case where MgZnCdSe is used as an n-type cladding layer. As a result, as the active layer, ZnsCd1−sSe (0<s<1) (hereinafter simply referred to as “ZnCdSe”) is used, and as the p-type cladding layer, a MgSe/BeZnTe laminate structure formed by alternately laminating a BetZn1−tTe layer (0<t<1) (hereinafter simply referred to as “BeZnTe”) and a MgSe layer is used, thereby 77 K oscillation of an yellow-green LD with a wavelength of 560 nm is achieved. The 77 K oscillation means that a light-emitting device is oscillated in a state in which the light-emitting device is cooled to 77 K. Moreover, as the active layer, instead of ZnCdSe, BeuZn1−uSewTe1−w (0<u<1, 0<w<1) (hereinafter simply referred to as “BeZnSeTe”) is used, thereby single peak light emission from orange to yellow-green at 594 nm, 575 nm and 542 nm is observed, and a 575-nm LED achieves light emission for 5000 hours or over at room temperature.
In this case, BeZnTe used as a part of the p-type cladding layer is lattice-matched to the InP substrate in the case where the Be composition ratio t is approximately 0.5, and the p-type hole concentration at this time is 1×1018 cm−3 or over (in actual, approximately 4.8×1018 cm−3) in the case where N is an acceptor, and a direct transition energy gap at a Γ point at this time is 3.12 eV at room temperature (an indirect transition energy gap is 2.77 eV at room temperature). In addition, BeZnTe has crystal strength unique to a Be chalcogenide material. On the other hand, ZnCdSe used as the active layer is lattice-matched to the InP substrate in the case where the Zn composition ratio s is approximately 0.48, and the energy gap at this time is 2.06 eV at room temperature.
Thus, the energy gap of BeZnTe is larger than that of ZnCdSe, so it is expected from a typical tendency in the past that the refractive index of BeZnTe is smaller than the refractive index of ZnCdSe; however, it is found out by an actual evaluation that the refractive index of BeZnTe is larger than the refractive index of ZnCdSe. In other words, it is found out that it is difficult to confine light within the active layer.
Moreover, when the band structures of ZnCdSe and BeZnTe are examined, it is found out that the top of the valence band of BeZnTe is higher than that of ZnCdSe, and a Type II junction between ZnCdSe and BeZnTe is formed. In other word, it is found out that they are in a state in which hole injection from the p-type cladding layer into the active layer is not sufficiently performed.
Therefore, the inventors of the present invention have focused attention on MgSe which has a distinctly small refractive index among materials relating to Group II-VI compound semiconductors, and has a lattice constant close to that of InP, and the inventors of the present invention have had an attempt to apply a MgSe/BeZnTe superlattice structure formed by alternately laminating BeZnTe and MgSe with a thickness corresponding to several molecular layers to the p-type cladding layer. As a result, it is found out that the refractive index of the p-type cladding layer becomes smaller than the refractive index of a ZnCdSe active layer (refer to I. Nomura et al. “Refractive Index Measurements of BeZnTe and Related Superlattices on InP and Application for Waveguide Analysis of MgZnCdSe/BeZnTe Visible Lasers” phys. stat. sol (b) 229, 987 (2002)). Moreover, a quantum confinement effect shifts a sublevel in the valence band, so the top of the sublevel in the valence band of the superlattice structure is expected to be lower than that of ZnCdSe, that is, it is expected that a Type I junction is formed (refer to broken lines in FIG. 6). FIG. 6 schematically shows an example of the band structure of each layer, and in FIG. 6, reference numerals 112, 113 and 115, 114, 116, 116A, and 116B denote an n-type cladding layer, guide layers, an active layer, a p-type cladding layer, a BeZnTe layer, and a MgSe layer, respectively. Therefore, it is found out that when the above-described MgSe/BeZnTe superlattice structure is applied to the p-type cladding layer, it becomes possible to sufficiently perform hole injection from the p-type cladding layer into the active layer and to confine light within the active layer.
However, it is reported that MgSe has high reactivity, and doping into MgSe is expected to be relatively difficult, and when MgSe has a thickness of approximately 0.2 to 0.4 μm, MgSe is changed from a zinc blend (ZB) structure to a rock salt (RS) structure, and the lattice constant of MgSe is also changed, so a MgSe single layer structure has not been examined in depth (refer to H. M. Wang et al. “Surface reconstruction and crystal structure of MgSe films grown on ZnTe substrates by MBE” J. Crystal Growth 208, 253 (2000)).